This invention relates to the electroding of poly(vinylidene fluoride) and more particularly to the surface treatment of poly(vinylidene fluoride) and the selection of metallization steps for forming electrodes on the surface of the poly(vinylidene fluoride).
Poly(vinylidene fluoride) is a polymer which has been found to exhibit "piezoelectric" properties when suitably processed. The term "piezoelectric " is used herein to denote the property of a material by which the application of a mechanical stress to the material produces an electrical potential across the material. The term also connotes the converse property (which is also present to some degree) by which application of an electrical potential through the material produces a stress and resultant mechanical strain. The poly(vinylidene fluoride) material has been found to exhibit the piezoelectric property only after non-random or or crystalline molecular orientations have been created, and after polarization has occurred in respect to these oriented molecular constituents. The orientation and polarization of the poly(vinylidene fluoride) takes place at room temperature and above, and may be destroyed if care is not exerted in subsequent treatment of the material. Exposure to excessive electric fields or excessive temperature may have this effect. Given that the material has suitable piezoelectric properties for use, it is essential that the electrodes withstand exposure to pressure and provide the quality of electrical contact required. One application of this piezoelectric material is the sensing of acoustic vibrations in sea water, typified by a hydrophone.
Accordingly, it is an object of the invention to provide a novel method of electroding a poly(vinylidene fluoride) solid.
It is another object of the invention to provide a novel method of electroding a poly(vinylidene fluoride) solid in a piezoelectric application, having improved adhesion.
Another object of the invention is to provide a novel method of electroding a poly(vinylidene fluoride) solid in a piezoelectric application having improved adhesion and facilitating optimum piezoelectric properties.
These and other objects of the invention will be dealt with in the description which follows. They are achieved in a novel method of electroding a poly(vinylidene fluoride) (PVF.sub.2) solid comprising the steps of abrading the surface of the PVF.sub.2 solid to provide an irregular bonding surface, rinsing the surface to remove surface contaminants; plasma etching the surface by activated free radical and ionic species of oxygen, to create temporary molecular scale surface irregularities (while keeping the PVF.sub.2 solid at a temperature below the melting point of its crystallites for dimensional stability or below the temperature for reversion to the unpolarized state), and sputtering a metallic layer on the prepared surface of the PVF.sub.2 solid before the prepared surface has deteriorated.
In a practical embodiment, the initial sputtered metallic layer is chromium, followed by a sputtered copper layer, which is then used as the basis for electrodeposition of a more substantial copper layer. The copper layer may be the final element of the metallization, or it may be followed by an electrodeposited nickel layer, or an evaportaed gold layer. In practice, all steps in the process and in particular the plasma etching, sputter etching, and sputtering steps may be conducted at a temperature which does not adversely affect the piezoelectric properties of the poly(vinylidene fluoride).